Zeolite is a kind of silicate having a reticulate crystal structure in which fine pores of uniform diameter are formed. It is known that zeolite has various chemical compositions represented by a general formula of WmZnO2n·sH2O (W: sodium, potassium, calcium or the like; Z: silicon, aluminum or the like; s: various values) and has crystal structures of many kinds (types) different in pore shape. These zeolites have inherent absorbabilities, catalytic activities, solid acid characteristics, ion exchange abilities, etc. based on respective chemical compositions and crystal structures, and are used in various applications such as adsorbent, catalyst, catalyst carrier, gas separation membrane, and ion exchanger.
An MFI type zeolite, for example, is a zeolite having pores of about 0.5 nm formed by the oxygen-containing ten-membered ring in the crystal and is used generally in applications such as adsorbent for adsorbing harmful substances present in automobile exhaust gas, such as nitrogen oxides (NOx), hydrocarbons (HC) and the like, catalyst for decomposing such harmful substances, and the like.
Zeolite is ordinarily powdery or particulate. In order to use zeolite in the form of a separation membrane, there is being developed methods for forming a zeolite membrane. A zeolite membrane is obtained, for example, by subjecting zeolite raw materials to a hydrothermal synthesis to precipitate zeolite crystals on the surface of a support in a membrane state. Here, the hydrothermal synthesis refers to a method for heating zeolite raw materials in the presence of water (steam) under given conditions to synthesize zeolite.
In such production of a zeolite membrane, mere hydrothermal synthesis in the presence of a support has had problems of exposure of support surface and too large membrane thickness (which made difficult the formation of dense and thin membrane). There were disclosed methods for solving the above problems by coating a seed crystal on the surface of a support (see, for example, patent literatures 1 to 3).
There were also disclosed methods of immersing a support in an aqueous solution containing zeolite raw materials, to precipitate a seed crystal on the surface of the support (see, for example, patent literatures 4 and 5).
Patent literature 1: JP-A-H10-36114
Patent literature 2: JP-A-2002-201020
Patent literature 3: JP-A-2004-82008
Patent literature 4: JP-A-2004-307296
Patent literature 5: JP-A-2000-26115